1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a technique which is effective when applied to a TFT (Thin Film Transistor) liquid crystal display device including poly-silicon transistors.
2. Related Art
As one liquid crystal display device of the prior art, there is known an active matrix liquid crystal display device which has active element in each pixel and in which the active elements are used for switching operation.
The feature of the active matrix liquid crystal display device is that since liquid crystal driving voltages (or gradation voltages) are applied to pixel electrodes through the active elements such as thin film transistors (TFT) or the like, no crosstalk occurs between the pixels. As a result, any special drive method for preventing the crosstalk as in the simple matrix liquid crystal display device need not be used, making multiple gradation display possible.
As one of the types of this active matrix liquid crystal display device, there is known a TFT active matrix liquid crystal display device which uses thin film transistors such as amorphous silicon transistors or poly-silicon transistors as the active elements.
Hereinafter, the amorphous silicon transistor will be referred to as the xe2x80x9ca-SiTrxe2x80x9d, the poly-silicon transistor as the xe2x80x9cP-SiTrxe2x80x9d, the TFT liquid crystal display device using amorphous silicon transistors as the xe2x80x9ca-SiTFT liquid crystal display devicexe2x80x9d, and the TFT liquid crystal display device using the poly-silicon transistors as the xe2x80x9cP-Si-TFT liquid crystal display devicexe2x80x9d.
The a-SiTFT liquid crystal display device is widely used as the display device for a personal computer and a TV set.
In the a-SiTFT liquid crystal display device, however, a drive circuit for driving the liquid crystal needs be disposed in. the periphery of the liquid crystal display panel.
In recent years, on the other hand, the TFT liquid crystal display device using P-SiTr elements has been developed and used in a liquid crystal projector or a head-mounted (glasses type) display.
In this liquid crystal display panel of the P-SiTFT liquid crystal display device, as in the liquid crystal display panel of the a-SiTFT liquid crystal display device, the P-SiTr are arranged and formed in a matrix over a quartz or glass substrate.
Since the operating speed of the P-SiTr is higher than that of the a-SiTr, moreover, the peripheral circuit of the liquid crystal panel of the P-SiTFT liquid crystal display device can also be mounted on the common substrate.
Generally in the liquid crystal projector, the image is displayed by irradiating the liquid crystal display panel with light emitted from a light source(Lamp), by controlling the irradiating light to pass the liquid crystal display panel by means of the image formed on the liquid crystal display panel, and by projecting the control light onto the screen.
The thin film transistor disposed at each pixel is caused to malfunction by photoconduction caused when the transistor is irradiated with light.
In the liquid crystal panel of the TFT liquid crystal display device used in the liquid crystal display projector of the prior art, therefore, a light shielding film is provided on the light source side to prevent light from falling directly on the thin film transistors.
However, the image of higher brightness has been demanded as the image to be displayed by the liquid crystal display projector, so that the illuminance of the light source has a tendency to increase (to about 10 million luxes in the future).
As the illuminance of the light source increases, moreover, in the liquid crystal display panel of the TFT liquid crystal display device used in the liquid crystal display projector, there arises a problem that the thin film transistors are caused to malfunction not only by the light coming from the light source directly and entering into the thin film transistors but also by reflected light entering into the thin film transistors from the display face side opposed to the light source side.
Representative aspects of the invention to be disclosed herein will be briefly summarized in the following.
According to one aspect of the invention, there is provided a liquid crystal display device comprising:
opposing two substrates, at least one which is transparent, a liquid crystal layer interposed therebetween;
pixels formed over at least one of the two substrates and arranged in a matrix;
pixel electrodes and semiconductor elements provided in said pixels; and
wherein each of said semiconductor elements includes a semiconductor layer with the first surface and the second surface, a first gate electrode located at the side of the first surface of said semiconductor layer, and a second gate electrode located at the side of the second surface of said semiconductor layer, and
wherein said second gate electrode shades the light from the side of the second surface into the said semiconductor layer.
According to another aspect of the invention, there is provided a liquid crystal display device comprising:
a liquid crystal panel having a plurality of pixels;
pixel electrodes provided in the pixels ;
semiconductor elements provided in the pixels and having a semiconductor layer, a first control electrode and a second control electrode;
wherein said semiconductor layer formed between said first control electrode and said second control electrode; and
each of said semiconductor elements is controlled by said first control electrode and said second control electrode to apply video signals to said pixel electrodes.
According to still another aspect of the invention, there is provided a liquid crystal display device comprising:
a first substrate and a second substrate;
a liquid crystal sandwiched between said first substrate and said second substrate;
a plurality of pixels formed over said first substrate;
semiconductor elements provided in the pixels;
a semiconductor layer constituting said semiconductor elements;
a gate electrode formed to the liquid crystal side against a semiconductor layer; and
a light shielding film formed to the first substrate side against a semiconductor layer,
wherein said semiconductor elements are controlled by control signals applied to said gate electrode and said light shielding film.
An object of the invention is to provide a technique of preventing the semiconductor elements of a liquid crystal display device from malfunctioning due to the light which falls on the semiconductor elements from the display face side opposed to the light source side.
Another object of the invention is to provide such a technique of decreasing the leakage current of the semiconductor elements provided in every pixels in a liquid crystal display device and increasing the ON current.
The aforementioned and other objects and novel features of the invention will made apparent from the following description, to be made with reference to the accompanying drawings.